The present invention relates to a circuit arrangement for determining the connection condition of a subscriber line in a telecommunications network. The subscriber line comprises two wires which extend between a subscriber station, such as a telephone, and a subscriber line interface circuit which may, for example, form part of a private branch exchange. The two wires of the subscriber line are connected in the subscriber line interface circuit via respective resistors to the two poles of a DC voltage source. The connection condition of the subscriber line (e.g., on-hook, off-hook and ground key actuation) is determined by evaluating the potentials which exist at sampling points within the subscriber line interface circuit. The determination is made, from among the various possible connection conditions of the subscriber line, in dependence upon the operating condition of the subscriber line interface circuit.
In telephone exchanges, a subscriber line interface circuit is required at the point of connection between the subscriber line and the exchange. This subscriber line interface circuit performs a number of functions. It serves, inter alia to transmit the speech signal in both directions, to supply the calling signal to the line and to form a terminal impedance for the line. In addition, the subscriber line interface circuit contains a two-to-four wire hybrid for transforming the two wire system of the subscriber line into the four wire system within the exchange. In exchanges which contain an electronc switching matrix, the subscriber line interface circuit contains a device for supplying a DC voltage to the telephone subscriber station. This voltage supply device comprises supply resistors over which each wire of the subscriber line is connected with the supply source. In this way, the changes in potential which occur as a result of changes in the connection conditions of the line may be used in the determination of these connection conditions (open loop, closed loop and ground key actuation). An evaluating circuit coupled to the corresponding wires of the subscriber line is therefore able to recognize a request for a line connection initiated at the subscriber station or a ground key actuation at the subscriber station signalling a command to initiate a call-back or transfer procedure. When the handset at the subscriber station is taken off-hook and closes the subscriber line loop, it must be determined whether a subscriber station is being called, and the dialing pulses from the calling station must be evaluated. With this particular evaluation, it is necessary to provide a sufficient margin of safety against erroneous operation. This is made difficult by the fact that a certain leakage current can flow even in the open-loop (on-hook) condition of the line. In addition, interference can occur due to AC voltage components which are induced in the line. The evaluation circuit must be designed so that it can determine the connection condition of a subscriber line independently of such outside influences.
If the DC voltage supply device for the two subscriber line wires is not exactly symmetrical in construction, the equalizing currents which result from this lack of symmetry can also have an interfering effect.
In addition to the requirement of a sufficient margin of safety in the determination of the connection condition, it is necessary, particularly with electronic exchanges, to minimize the cost of the functional units coupled with the individual subscriber line interface circuits. In this way, it is possible to combine a large number of such units within a single sub-component of the exchange.